Available evidence provides a support for the idea that substance P, gramma-amino-butyric acid and somatostatin are involved in synaptic transmission processes in the superficial layers of the spinal dorsal horn. However, the identity of the neural elements responsive to these agents, and the ionic mechanisms underlying atheir synaptic actions are not as yet clear. The intent of this proposal is to identify the elements these putative transmitters act upon, determine their membrane effects and try to explain the mechanisms of their actions. Conventional approach will be to use cat spinal cord in situ and apply GABA, somatostatin and substance P iontophoretically or by micropressure injections into the laminae I-III of the dorsal horn, while performing extra - and intracellular recordings from the large diameter primary afferents or dorsal root ganglion cells. Ionic mechanisms of presynaptic and postsynaptic actions of GABA, somatostatin and substance P will be investigated in virtro by using the rat spinal cord slice preparation. Morphological identification of physiologically and pharmacologically characterized dorsal horn interneurons and primary afferent neurons will be done by intracellular labeling with horseradish peroxidase.